Encoder apparatus

ABSTRACT

An encoder apparatus comprising a scale and a readhead assembly comprising a scale signal receiver. The scale and the scale signal receiver are located within a protective housing which is configured to protect them from contamination located outside the protective housing and comprises a seal through which the scale signal receiver can be connected to a part outside the protective housing. The arrangement of the scale signal receiver inside the protective housing is independent of the scale and protective housing.

The present application is a continuation of U.S. patent applicationSer. No. 15/775,961, filed on May 14, 2018, which is a national stageentry of PCT/GB2016/053779, filed on Dec. 1, 2016, which claims priorityto European Patent Applications 15275246.5 and 15275245.7, both of whichwere filed on Dec. 3, 2015, the disclosures of each of which are herebyincorporated by reference.

This invention relates to an encoder apparatus. For instance, theinvention relates to what is commonly known as an enclosed encoder, alsocommonly known as a sealed encoder.

Encoders are used in many industries to provide position (or itsderivatives, e.g. velocity and/or acceleration) feedback to a controlsystem of a machine, e.g. feedback control for the position/motion ofone part of a machine relative to another part of the machine. As willbe understood, typically a scale is provided on one part of the machineand a readhead for reading the scale is provided on the other part ofthe machine such that the relative position of scale and readhead, andhence the relative position of the machine parts, can be detected by thereadhead along the encoder's measurement dimension.

The technologies utilised by such encoders can require that theenvironment in which they are used is clean and free of contamination,e.g. dust, dirt and moisture (which could, for example, be oil and/orwater based). Contamination on the scale and/or readhead can adverselyaffect the performance of the encoder. In many industries such machinesthat use encoders operate in an appropriately clean environment, inwhich case what is commonly referred to as an “exposed encoder” (or“open encoder”) can be used.

However there are instances, such as in the machine tool industry forexample, where the working environment is not clean, and where fluidsand solid debris are prevalent. In such cases there exist encoders whichare protected against such detrimental environments. Typically, in thesecircumstances, sealed (also known as enclosed) encoders are used.

An example of a sealed encoder module 2 is schematically illustrated inFIGS. 1a to 1d . As illustrated, the sealed encoder module 2 comprises ascale 4 and a readhead assembly comprising a scale signal receiver 6.The scale 4 and the scale signal receiver 6 are located inside aprotective housing 8 which protects them from contaminants external tothe protective housing. The scale 4 is fixed to the protective housing 8whereas the scale signal receiver 6 of the readhead assembly can movealong the length of the scale 4 within the protective housing 8. In use,the protective housing 8 will be secured to a first part of a machine(not shown) and the readhead assembly will be secured to a second partof the machine, which is moveable relative to the first part along the xaxis. In practice, during use, the first part of the machine (and hencethe protective housing/scale) could be configured to move, and/or thesecond part of the machine (and hence the readhead) could be configuredto move.

The readhead assembly comprises a mounting block 14 which is to bedirectly fastened to the second part of the machine (e.g. via boltspassing through bolt holes 15 in the mounting block 14), a blade 16 andan articulated linkage 18 which connects the scale signal receiver 6 tothe blade 16 (described in more detail below).

The protective housing 8 further comprises a seal in the form of a pairof sealing lips 12 which seals the inside of the protective housing 8,in which the scale 4 and scale signal receiver 6 reside, from externalcontaminants. The blade 16 passes through the seal (between the pair ofsealing lips 12) and the sealing lips 12 allow the movement of the blade16 and hence the scale signal receiver 6 along the length of theprotective housing 8/scale 4.

The position of the scale signal receiver 6 relative to the scale 4 inall degrees of freedom other than along the length of the scale istightly controlled by bearings 20 (e.g. roller bearings) in the scalesignal receiver 6 which engage and bear against the scale 4 (but as willbe understood could additionally/alternatively bear against the insideof the protective housing). Springs (not shown) bias the scale signalreceiver's bearings 20 against the scale 4. Any misalignment in the axisof the first and second parts of the machine is accommodated by thearticulated linkage 18. In this embodiment, the articulated linkage 18is provided by a joint, which includes at least one pivot joint. Thearticulated linkage permits pitching, rolling and yawing (i.e.rotational movement about three mutually perpendicular axes) of thescale signal receiver 6 relative to the mounting block 14, as well aslateral motion of the scale signal receiver 6 relative to the mountingblock 14 in directions perpendicular to the measuring dimension (lengthof the scale). Accordingly, other than along the measuring dimension(along the x axis in the shown embodiment), the position and motion ofthe scale signal receiver 6 is constrained by the scale 4. In otherwords, the scale signal receiver 6 is guided by the scale 4. Thearticulated linkage 18 therefore decouples the scale signal receiver 6and mounting block 14 in all degrees of freedom other than along thedimension of measurement of the encoder apparatus (which should becoincident with the direction of motion of the first and second parts ofthe machine), which in the embodiment shown in FIG. 1 is along the xaxis. This is the sort of encoder apparatus that is disclosed in U.S.Pat. No. 4,595,991.

As also shown in FIG. 1b , a power/communications cable 5 can beprovided to enable the readhead assembly to be powered and facilitatecommunication between the readhead assembly and an external processordevice (e.g. a machine controller). Furthermore, an air supply line 9can be provided for supplying air into the protective housing 8, so asto create a positive pressure within the protective housing 8.Accordingly, in the case that the sealing lips 12 do not form a perfectseal (in particular where the lip seals are parted by the blade 16) airwill tend to flow out of the protective housing 8 due to the positivepressure. The positive pressure thereby provides further resistance tophysical contamination trying to enter the protective housing 8. As willbe understood such contamination can comprise solid and/or fluidcontamination, examples of which include swarf, liquid (e.g. coolant)and/or air-borne moisture. As also shown, another air supply line 7which supplies air into the protective housing via the readhead assembly(e.g. via a conduit passing through the mounting block 14 and blade 16)can be provided.

The present invention provides an improved encoder apparatus. Inparticular cases, the present invention relates to improvements tosealed encoders. For instance, according to the present invention thereis provided a sealed encoder module comprising a scale, a readhead and aprotective housing (e.g. an integral protective housing). In particular,aspects of the invention described herein relate to an improved encoderof the type where the readhead's scale signal receiving part is locatedon a first side of a seal and the readhead's mounting part is located ona second side of the seal.

According to a first aspect of the invention there is provided anencoder comprising a scale and a readhead assembly moveable relative toeach other. The readhead assembly can comprise a scale signal receiver.The scale and the scale signal receiver can be located within aprotective housing which is configured to protect them fromcontamination located outside the protective housing. The protectivehousing can comprise a seal through which the scale signal receiver canbe connected to a part outside the protective housing. The arrangementof the scale signal receiver inside the protective housing can beindependent of the scale and protective housing.

Whilst the enclosed encoder of the sort shown in FIG. 1 provides aguaranteed relationship of the scale and scale signal receiver (e.g.guaranteed ride-height), such an arrangement of an articulated linkage,with rolling/sliding element bearings can have detrimental effects, suchas hysteresis position errors caused by friction and linkage compliance,and position errors caused by for example pitching errors of the scalesignal receiver caused by bearing runout errors or dirt under thebearings. This aspect of the invention relates to a sealed/enclosedencoder that removes such hysteresis and position errors by making the(physical) arrangement of the scale signal receiver inside theprotective housing independent of the scale and protective housing (thatis the physical relationship of the scale signal receiver with respectto the scale is independent). In other words, the arrangement of thescale signal receiver inside the protective housing can be independentof the scale and protective housing in at least one degree of freedomother than in the measuring degree of freedom. As will be understood, adegree of freedom could be a rotational or a linear degree of freedom.Preferably, the arrangement of the scale signal receiver inside theprotective housing is independent of the scale and protective housing inall linear and rotary degrees of freedom. Accordingly, the enclosedencoder of the invention can be provided without any bearings betweenthe scale signal receiver and the scale and/or protective housing. Inother words, the enclosed encoder of the invention can be providedwithout any bearings that engage and/or constrain the scale signalreceiver to the scale and/or protective housing. Accordingly, thearrangement of the scale signal receiver within the protective housingis constrained independently of the scale and protective housing, and inother words is not constrained by the scale or protective housing. Thisavoids the need for an articulated linkage. Rather, the presentinvention relies on external means for guiding the scale signal receiverrelative to the scale. Accordingly, as will be understood, the scalesignal receiver could be described as being externally constrained, asbeing unguided, as being without integral bearing, or as beingbearingless. Another way of looking at this is that the scale signalreceiver is held suspended (in other words in a suspended state) withinthe protective housing.

As will be understood, the scale signal receiver and the protectivehousing are moveable relative to each other along the measuringdimension of the scale. Accordingly, as will be understood, the scalesignal receiver is located within (and protected by) the protectivehousing, but not mounted to the protective housing.

As will be understood, the seal permits relative movement of the scalesignal receiver and the protective housing along the measuring dimensionof the scale. Accordingly, as described in more detail below, the sealextends along the measuring dimension. The seal could also accommodatesome relative movement of the scale signal receiver and the protectivehousing in other dimensions.

The encoder can be what is commonly referred to as a sealed encoder(also commonly known as an enclosed encoder). These can also be known asa sealed (enclosed) encoder module.

The protective housing can be an integral part of the encoder.Optionally, the scale is mounted to the protective housing. The encodercould be configured such that the scale is configured to be mounted to apart of a machine (the position of which is to be measured by theencoder) via the protective housing. That is the protective housingcould comprise one or more mounting features via which the scale isconfigured to be mounted to a part of a machine. Accordingly, optionallythe protective housing can lie between the scale and the part of themachine that the encoder module is configured to be mounted to. As willbe understood, the protective housing could be configured such that inuse it is a single fixed unit (i.e. it does not comprise parts whichmove relative to each other, e.g. with the movement of the relativelymoveable parts of the machine on which it is mounted).

As will be understood, the scale signal receiver can be the part of thereadhead assembly located inside the protective housing which receivesthe signal from the scale. The scale signal receiver can comprise one ormore components for interacting with the scale signal, e.g. so as todetect the scale signal and/or manipulate the scale signal before it issubsequently detected. For example, in the case of an optical encoder,the scale signal receiver can comprise one or more optical elements,such as diffractive and/or refractive optical elements. For example, thescale signal receiver can comprise one or more lenses, and/or one ormore diffraction gratings. The scale signal receiver could comprise oneor more signal guides for guiding the scale signal to another component.For example, in the case of an optical encoder, the scale signalreceiver could comprise a wave guide, e.g. a light guide (for instance,an optical fibre). The signal guide could be configured to carry thescale signal to a subsequent component which interacts with the scalesignal, e.g. so as to manipulate the scale signal. The signal guidecould be configured to carry the scale signal to one or moredetectors/sensors configured to detect the scale signal, e.g. atransducer.

Optionally, the readhead assembly comprises one or more sensors forsensing the scale signal (which as described above may or may not havebeen manipulated by one or more components in the readhead assembly).The sensor could comprise a plurality of sensor elements, e.g. an arrayof sensor elements. The scale signal receiver could comprise thesensor(s). Optionally, the sensor could be located elsewhere in thereadhead assembly. For example, the sensor could be located in a part ofthe readhead assembly which is located outside the protective housing.For example, in those embodiments in which the readhead assemblycomprises a mounting block (described in more detail below), the sensor(and indeed any other components mentioned above) could be located inthe mounting block.

In those embodiments in which the scale signal receiver comprises anouter casing (described in more detail below), the scale signal receivercan comprise one or more features for enabling the signal from the scaleto enter the scale signal receiver. For example, in the case of anoptical encoder, the scale signal receiver could comprise a window.

The readhead assembly can comprise one or more emitters for emittingenergy toward the scale. For example, the readhead assembly can compriseat least one light source configured to illuminate the scale (e.g. withlight in the infra-red to ultraviolet range). The scale signal receivercan comprise said one or more emitters. Optionally, said one or moreemitters can be provided by another part of the readhead assembly (e.g.outside the protective housing, such as provided by a mounting block).

Optionally, the readhead, e.g. the scale signal receiver, (for instanceits sensor(s)) is configured to detect a signal generated by lightcoming from the scale. Optionally the light has been transmitted throughthe scale. Optionally, the light has been reflected from the scale.Accordingly, optionally, the readhead, e.g. the scale signal receiver,comprises an emitter (e.g. a light source) and a sensor. The emitter andsensor could be located on the same side of the scale. Accordingly, theencoder can be a reflective encoder apparatus.

As will be understood, the scale will have some form offeatures/markings which can be read by the readhead to determinedisplacement, position (or its derivatives, e.g. velocity and/oracceleration). Such features could define a pattern. For example, anincremental scale could comprise scale features/marks that define aperiodic pattern and which can be used to generate a periodic signal atthe readhead (e.g. when relative movement between the scale and thereadhead takes place). The scale can be elongate. The scale can comprisea substrate in and/or on which the features/markings are formed.

Optionally, the encoder apparatus is a diffraction-based encoderapparatus. Optionally, the scale comprises features configured todiffract light, which is then used to form a resultant signal on asensor in the readhead assembly. Optionally, the readhead assemblycomprises one or more optical elements configured to interact with lightbefore and/or after the scale in order to form the signal on a sensor inthe readhead assembly. Optionally, the readhead assembly comprises oneor more lenses and/or one or more diffraction gratings. Optionally, thereadhead assembly comprises a diffraction grating configured to interactwith light from the scale to form an interference fringe on a sensor inthe readhead assembly. Optionally, the sensor comprises anelectrograting comprising two or more sets of interdigitated sensors,each set being configured to detect a different phase of an interferencefringe.

Optionally, the scale comprises absolute scale features which define a(e.g. continuous) series of uniquely identifiable positions along thelength of the scale.

Optionally, the readhead assembly is configured to detect an image ofthe scale. Optionally, the readhead assembly (e.g. the scale signalreceiver) comprises at least one imaging optical element configured toform an image of the scale onto a sensor. Optionally, the readheadassembly comprises one or more sensors suitable for capturing an image,e.g. one or more Charge-Coupled Devices (CCD) or ComplementaryMetal-Oxide-Semiconductor (CMOS) sensors.

As will be understood, references to “optical” and references to “light”are intended to refer to electromagnetic radiation (EMR) in theultraviolet to infra-red range (inclusive).

As will be understood, the readhead assembly can be configured todetermine and output information concerning the relative position of thescale signal receiver and the scale (referred to herein as “positioninformation”). Optionally, the readhead assembly comprises one or moreprocessor devices configured to process the output from one or moresensors/detectors, e.g. so as to form said position information. Theposition signal can be incremental position information. For example,the position signal can comprise a quadrature signal. Optionally, theposition signal comprises absolute position information. Said one ormore processor devices could be located in the scale signal receiverand/or in another part of the readhead assembly (e.g. in the readheadmount).

As will be understood, the part outside of the protective housing towhich the scale signal receiver is configured to be connected, could bepart of a machine, the position/movement of which relative to anotherpart of the machine (to which the scale is secured) is to be determined.

Since the arrangement of the scale signal receiver inside the protectivehousing is independent of the scale and protective housing, preferablythe scale signal receiver is configured to be rigidly connected to saidpart outside the protective housing. The scale signal receiver can beconfigured to be connected to a part of a machine located outside theprotective housing, e.g. via a mount member. Accordingly, the mountmember could be a rigid mount member. Accordingly, said rigidconnection/rigid mount member can be configured such that the positionand orientation of the scale signal receiver within the protectivehousing, in all six degrees of freedom, can be dictated by (and masteredto) the part outside the protective housing to which the scale signalreceiver is configured to be attached to. For example, in embodiments inwhich the readhead assembly comprises mounting features (describedbelow), the position and orientation of the scale signal receiver withinthe protective housing, in all six degrees of freedom, can be dictatedby (and mastered to) the mounting features (e.g. dictated by/mastered toa mounting block on which said mounting features are provided).

For example, the scale signal receiver can be rigidly fixed to a rigidreadhead mount member which passes through the seal. Accordingly, theposition and orientation of the scale signal receiver on the first sideof the seal (inside the protective housing) can be dictated by (andmastered to) the readhead mount.

Said mount can be provided by the part of the machine to which the scalesignal receiver is to be attached. For example, the machine itself couldcomprise a (rigid) mounting bracket that is inserted into the protectivehousing and connected to the scale signal receiver. Optionally, thereadhead assembly can comprise a readhead mount comprising one or moremounting features located outside the protective housing for securingthe readhead assembly to a part of a machine. As will be understood, thereadhead assembly could be configured to be releasably fastened to apart of a machine. The one or more mounting features could be providedon a mounting block. A mounting feature could comprise, for example, ahole into and/or through which a releasable fastener (e.g. a bolt) canpass (and optionally engage). As will be understood, the scale signalreceiver of the readhead assembly can be rigidly connected to thereadhead mount (which as explained above can be rigid so as to ensure arigid connection between the scale signal receiver and a part outsidethe protective housing). As will be understood, the scale signalreceiver, readhead mount and blade could be formed as a singlemonolithic structure, or could comprise a plurality of separately formedunits, rigidly connected to each other.

The mount could comprise a (rigid) blade-like member configured toextend through the seal. In those embodiments in which the readheadassembly comprises the readhead mount as described above, the blade-likemember could extend through the seal between the scale signal receiverthat is located inside the protective housing and the mounting featuresthat is located outside the protective housing. The blade-like membercould comprise first and second edges (in other words, leading andtrailing edges). The blade-like member could be tapered towards thefirst and second edges. The blade-like member could comprise an internalpassageway/channel for wires and/or air to pass through between theinside and outside of the protective housing, for example between thescale signal receiver and a mounting block on which the one or moremounting features are provided.

The protective housing can comprise one or more mounting features formounting the protective housing to a part of a machine (e.g. todifferent part of the machine to which the scale signal receiver isconfigured to be mounted to, said parts of the machine being relativelymovable with respect to each other). Said one or more mounting featurescould be configured to facilitate releasable mounting of the protectivehousing. A mounting feature can comprise a hole into and/or throughwhich a releasable fastener (e.g. a bolt) can extend (and optionallyengage).

The encoder apparatus could comprise a magnetic, inductive, capacitive,and/or optical encoder apparatus. Accordingly, the scale could comprisemagnetic, inductive, capacitive, and/or optical scale. Optionally, theencoder apparatus comprises an optical encoder apparatus.

The scale can comprise rotary scale. The rotary scale can comprise whatis commonly referred to as disc scale (in which the scale features areprovided on the face of the disc). The rotary scale can comprise what iscommonly referred to as ring scale (in which the scale features areprovided on the circumferential edge of the disc). Optionally, the scalecan comprise linear scale.

Optionally, the encoder module has a nominal ride-height of not lessthan 0.1 mm, for example not less than 0.2 mm, for instance not lessthan 0.5 mm. Optionally, the encoder apparatus has a nominal ride-heightof not more than 5 mm, for example not more than 2 mm, for instance notmore than 1 mm. Optionally, the allowable ride-height variation(“tolerance”) for the encoder module is not less than +/−50 μm(microns), optionally not less than +/−75 μm (microns), for example atleast +/−100 μm (microns).

The protective housing can be elongate. The protective housing can besubstantially straight. The protective housing can comprise asubstantially tubular form. The cross-sectional shape of said tubularprotective housing need not necessarily be round, but for example couldcomprise other regular or irregular shapes. For example, thecross-section shape of said tubular protective housing could besubstantially rectangular.

Said seal could be provided on a first side-wall of the protectivehousing. Optionally said seal is provided along an edge between twoside-walls of the protective housing. Said seal could be substantiallyelongate. The seal can extend along the encoder apparatus' measurementdimension. Optionally, the seal is provided by a flow of gas, e.g.across a gap in the protective housing, and/or for example via apositive (e.g. air) pressure inside said protective housing. Optionally,the seal comprises a physical barrier. The seal could comprise aplurality, for example a pair, of seal members. For example, the sealcould comprise a plurality (e.g. a pair) of sealing lips (e.g. whichcould be elongate or annular/ring-shaped). The member which connects thescale signal receiver to the part outside the protective housing couldpass through the seal, e.g. between the sealing lips. For example, theabove mentioned blade-like member could pass through the seal, e.g.between the sealing lips.

Optionally, the seal (e.g. the sealing lips) is (are) compliant.Optionally, the seal (e.g. the sealing lips) is (are) elastic. Forexample, the seal (e.g. the sealing lips) is (are) sufficientlycompliant so as to enable the relative movement of the scale/protectivehousing and the scale signal receiver (in particular by permitting themember, e.g. blade-like member, and the protective housing/seal to moverelative to each other). Optionally, the seal (e.g. the sealing lips) is(are) biased toward a sealed configuration, e.g. by way of theirelasticity. The seal (e.g. the sealing lips) could comprise, forexample, polyurethane, such as thermoplastic polyurethane, and/orfluorinated elastomer.

The readhead assembly can comprise at least one vibration controldevice. As will be understood, such one or more vibration controldevices can be configured to reduce the susceptibility of the of thereadhead assembly (e.g. of the scale signal receiver) to vibrations. Avibration control device can be a device configured to reduce theresponse of at least part of a system (e.g. of the scale signal receiverof the readhead assembly) due to external excitation. The at least onevibration control device can comprise at least one member which isconfigured to vibrate independently of the readhead assembly, e.g.independently of the scale signal receiver. As will be understood, thevibration control device can be configured to take energy out of avibrating readhead assembly/scale signal receiver. Optionally, thevibration control device is configured such that the resonancemagnification factor (also known as amplification factor) of thereadhead assembly/scale signal receiver is less than 50, for exampleless than 20, for instance less than 10.

The at least one vibration control device can comprise at least onemember which is configured with a resonant frequency independent of theparts of the readhead assembly that are located inside the protectivehousing (e.g. of the scale signal receiver). Optionally, the at leastone vibration control device comprises at least one member which isconfigured with a resonant frequency different to that of the parts ofthe readhead assembly that are located inside the protective housing(e.g. different to that of the scale signal receiver). The vibrationcontrol device can be associated with (e.g. coupled or connected to)only a single unitary/movable body (i.e. is not located between twoindependently moveable bodies). Accordingly, the vibration controldevice could be in contact with only a single unitary/movable body.

The at least one vibration control device could comprise a linearvibration control device. For example, it can comprise linear springstiffness. The at least one vibration control device could comprise anon-linear vibration control device. For example, it can comprisenon-linear spring stiffness.

The at least one vibration control device can be configured to controlvibrations in at least one degree of freedom, optionally in a pluralityof degrees of freedom, for example in at least one linear degree offreedom and at least one rotational degree of freedom. The at least onevibration control device can be configured to control one or more modesof vibration. For example, in the case of a tuned mass damper, the tunedmass damper can be tuned to multiple different resonant frequencies.

The at least one vibration control device can be configured to have amass which is at least 1% of the mass of the part of the readheadassembly which it is configured to control vibration of, optionally atleast 2% of the mass of said part, for instance at least 3% of the massof said part. The vibration control device can be configured to have amass which is not more than 30% of the mass of said part, optionally notmore than 25% of the mass of said part, for example not more than 20% ofthe mass of said part, for instance not more than 10% of the mass ofsaid part. As will be understood, said part can comprise the part of thereadhead assembly that vibrates in excess of the source vibration. Forexample, said part can be the part of the readhead assembly whichvibrates relative to the readhead mount. For instance, said part can bethat part/those parts of the readhead assembly which is/are locatedinside the protective housing. For example, said part can comprise thescale signal receiver.

The scale signal receiver and/or the member via which the scale signalreceiver can be attached to the part outside the protective housing(e.g. the readhead mount, in particular for example the blade-likemember) could comprise at least one vibration control device.

The at least one vibration control device could be located inside theprotective housing (e.g. on the scale signal receiver and/or the partthat extend through the seal such as the blade-like member). Forexample, the scale signal receiver could comprise the at least onevibration control device. The vibration control device can reside withinthe scale signal receiver. In particular, in embodiments in which thescale signal receiver comprises an outer case (as explained in moredetail below), the vibration control device can reside within said outercase (e.g. such that it is sealed from contamination). The vibrationcontrol device can reside within a void, e.g. a recess, for example ahole, provided by the scale signal receiver (e.g. provided by said outercase). The vibration control device can be configured so as to be ableto move (e.g. vibrate) within said void/hole independently of the restof the scale signal receiver/outer case. Optionally, the vibrationcontrol device could be provided on the outside of the scale signalreceiver/outer case.

The at least one vibration control device can comprise one or morespring elements. The at least one vibration control device can compriseone or more damper elements. Accordingly, the at least one vibrationcontrol device can comprise a vibration damping device. At least one ofthe one or more spring elements and at least one of the one or moredamper elements can be provided by a common/single/composite element,e.g. a spring damper element. A spring damper element can comprise anelastomer (e.g. rubber).

As will be understood, a damper element can comprise something thatconverts movement/motion energy into a different form, such as heat.Non-limiting examples of damper element include, for example, a viscousdeformable element (e.g. such as an elastomer material) or for exampletwo separate rigid or elastic elements configured to rub against eachother when exposed to vibrations.

The vibration control device can comprise a mass element. The masselement could be separate to the spring and/or damper element. Asexplained above, the mass element could be configured to have a specificmass with respect to the part of the readhead assembly which it isconfigured to control vibration of. At least one of the one or morespring elements, at least one of the one or more mass elements, and atleast one of the one or more damper elements can be provided by acommon/single/composite element, e.g. a spring mass damper element. Forexample, an elastomeric block.

The vibration control device can comprise at least one elastomericelement. For example, at least one elastomeric ring. Said elastomericelement could be mounted on a body of higher density than theelastomeric element.

The vibration control device can comprise a tuned mass damper. The tunedmass damper can be tuned so as to reduce the amplitude of vibrations inat least the part of the readhead assembly (e.g. of at least the scalesignal receiver) in which it is installed, at and around that part'sresonant frequency. A tuned mass damper can comprise at least one springelement. A tuned mass damper can comprise at least one damper element. Atuned mass damper can comprise at least one mass element. The at leastone spring's stiffness “k”, the at least one damper's dampingcoefficient “c” and the at least one mass's mass “m” can be selected (inother words “tuned”) so as to reduce the amplitude of vibrations of atleast the part of the readhead assembly (e.g. of at least the scalesignal receiver) in which it is installed, at and around that part'sresonant frequency.

A plurality of vibration control devices can be provided. As will beunderstood, different vibration control devices could be configureddifferently so as to reduce the amplitude of different resonantfrequencies. For example, different spring stiffness and/or differentmasses could be used. In those embodiments in which a damper element isalso provided, different damping coefficients could be used.

The encoder apparatus (e.g. a sealed encoder module) can be configuredto determine and output diagnostic information. As will be understood,the encoder apparatus/module (e.g. the readhead) can also configured todetermine and output information concerning the relative position of thescale and readhead (and hence the relative position of first and secondparts of a machine on which the encoder apparatus can be mounted) in ameasuring dimension/degree of freedom (which could be linear or rotaryfor example). The diagnostic information could be indicative of therelative arrangement of the scale and the scale signal receiver, inparticular in at least one dimension/degrees of freedom other than thatof the measuring dimension/degree of freedom of the encoder module.Accordingly, the diagnostic information could be dependent on therelative arrangement of the scale and the scale signal receiver, inparticular in at least one dimension/degrees of freedom other than thatof the measuring dimension/degree of freedom of the encoder module. Thesealed encoder module can be configured to determine and outputdiagnostic information regarding a scale signal detected by thereadhead. The scale signal could be the signal detected by one or moresensors (e.g. in the readhead) that are configured (and in use, used) todetect the scale so as to determine said measure of the relativedisplacement of first and second parts of the machine (in the measuringdimension/degree of freedom). The scale signal could be the detectedsignal from the scale that is used to determine said measure of therelative displacement of first and second parts of the machine. Thescale signal could be an incremental scale signal. Accordingly, thediagnostic information could be determined from the output of anincremental signal sensor of the readhead. The incremental scale signalcould be an interference fringe.

The scale signal could be a reference mark signal. Accordingly, thediagnostic information could be determined from the output of areference mark signal sensor of the readhead. The scale signal could bean absolute scale signal. The scale signal could be an image of thescale (e.g. a one dimensional or two dimensional image of the scale).Accordingly, the diagnostic information could be determined from theoutput of an image sensor of the readhead. In other words, thediagnostic information could be determined from an image (e.g. a one ortwo dimensional image) of the scale.

Optionally, the scale signal used to determine diagnostic information isnot the signal that is used to determine said measure of the relativedisplacement. Optionally, the scale signal from which diagnosticinformation is determined is detected by at least one sensor other thanthe sensor(s) the output of which is(are) configured to be used todetermine said measure of the relative displacement of first and secondparts of the machine. Such a sensor could be referred to as a“diagnostic sensor”. Accordingly, in other words, the encoder modulecould be configured such that output of the diagnostic sensor is notused to determine said measure of the relative displacement of first andsecond parts of the machine.

Accordingly, as explained in more detail below, the scale signaldetected by the readhead could be dependent on the relative arrangementof the scale and the scale signal receiver in at least onedimension/degree of freedom other than that of the measuringdimension/degree of freedom of the encoder module. As will beunderstood, the encoder module (e.g. the readhead) is also configured todetermine and output information concerning the relative position of thescale and readhead (i.e. position information, that is in the measuringdimension/degree of freedom). Accordingly, the encoder apparatus/modulecan be configured to determine and output both position and diagnosticinformation. Accordingly, the encoder apparatus (e.g. the sealed encodermodule) can comprise at least one processor configured to determine saiddiagnostic information.

As mentioned above, the diagnostic information could be dependent on(and hence indicative of) the relative arrangement of the scale and thescale signal receiver, in particular in at least one dimension/degree offreedom other than that of the measuring dimension of the encodermodule. For example, the diagnostic information could be dependent on(and hence indicative of) any one, any combination, or all, of the scaleand scale signal receiver's lateral positon, ride-height, pitch, roll oryaw, with respect to each other. Accordingly, for example, thediagnostic information could be dependent on (and hence indicative of)when the scale and the scale signal receiver are, or are not, in adesired relative arrangement in at least one degree of freedom otherthan that of the measuring dimension. Such arrangement information canbe particularly useful for those embodiments which have a scale signalreceiver which is arranged independently of the scale, as described inmore detail below.

As will be understood, the encoder apparatus/module could be configuredsuch that the diagnostic information determined and output can compriseinformation concerning the quality of the scale signal detected by thereadhead. The diagnostic information could provide a measure of thesuitability of the representation to provide position information; andin particular for example reliable and/or accurate position information.

Outputting said diagnostic information could comprise providing anoutput based at least in part on at least one parameter determined as aresult of a process configured to analyse the quality of the scalesignal. For example, the control of an output device, such as a visualoutput device, can be based on said at least one parameter. Optionally,the encoder apparatus/module is configured to output diagnosticinformation in the form of one or more human-detectable signals. Theencoder apparatus (e.g. the sealed encoder module) could comprise atleast one output device for outputting said diagnostic information as ahuman-detectable signal. Said output device can output a signalindicative of said diagnostic information. Said output device couldcomprise a visual output device. Said output device could be configuredto emit an optical signal. Optionally, the least one output device isprovided on said readhead. Optionally, said at least one output deviceis provided on said protective housing. As described in more detailbelow, the readhead could comprise a mounting block external to saidprotective housing for mounting the readhead to one of first and secondmoveable parts of a machine, and said output device can be provided onsaid mounting block.

The scale signal receiver can comprise an outer case. The outer case canbe configured to protect components of the scale signal receiver thatare located inside the protective housing from contamination (e.g. solidor fluid such as swarf or coolant, or for example moisture) that doeshappen to enter the protective housing. In particular, the outer casecan be configured to provide protection against fluid, for example,liquid. This can improve the reliability and longevity of the encoderapparatus. Said outer case could encapsulate said components. Saidcomponents can comprise electrical components, including any wiresand/or any printed circuit boards. Said components can comprise theabove described components which are configured to interact with thescale signal. The outer case can be a sealed body, for example ahermetically sealed case.

Accordingly, the sensor componentry of the scale signal receiver can becontained within a sealed body/outer case. In other words, the scalesignal receiver's electrical and/or other componentry used, for examplein the detection of the scale signal, can be contained within a sealedbody/outer case. For example, in the case of an optical encoderapparatus, optical components such as a lens, diffraction grating,beam-steering device or beam-divider can be contained with the sealedbody/outer case. The readhead's emitter (e.g. a light emitter) can becontained within the sealed body/outer case. A window (e.g. sealedwindow) in the sealed body/outer case can be provided for permitting thescale signal to enter the sealed body/outer case.

The outer case can be a rigid case. Such a rigid case can be configuredto protect the one or more components, (including any wires and/or anyprinted circuit boards) against solid objects which enter the protectivehousing. The outer case could be substantially box-like. For example, itcould have a generally rectangular cross-sectional profile. The outercase can provide a void/internal volume within which the one or morecomponents of the scale signal receiver are located. The outer case canprovide the structure (e.g. load bearing structure) to which one or morecomponents of the scale signal receiver are mounted. The outer case canbe (can be configured to be) mounted to one of the first and secondparts of the machine. This could be via the protective housing. Thiscould be via a readhead mount, e.g. as described above in connectionwith the other aspects of the invention. For example, the outer casecould be mounted to one of the first and second parts of the machine viaa mounting block. In embodiments in which the protective housingcomprises a seal through which the scale signal receiver can beconnected to a part outside the protective housing, the outer case cancomprise the part which extends through the seal. For example, inembodiments in which there is a blade-like member (as described above),the blade-like member can be part of the outer case. In particular, theblade-like member can contain and protect wires or otherelectrical/optical components from contamination which enters theprotective housing.

As will be understood, the outer case can comprise a plurality ofcomponents, e.g. a body and a lid, which together define an internalvolume within which the one or more components of the readhead arecontained.

The outer case can encapsulate at least all of the electroniccomponents, including any wires and any printed circuit boards, of thescale signal receiver which are located within the protective housing.In the case of an optical encoder, the outer case could encapsulate allof the optical components used in the detection of the scale signal(e.g. any combination of one or more lenses, diffraction gratings, beamsplitters, light sources, and beam steerers), except for an outer-sideof one or more windows through which the scale signal enters the casingand/or through which light from a light emitter exits the outer casetoward the scale. Accordingly, as will be understood, any such windowscan form part of the outer case. Optionally, any electronic componentthat comprises a protective shell or body (e.g. which shields the bareelectronics of the electronic component) can itself form part of theouter case.

The encoder apparatus could comprise a reflective optical encoderapparatus. In such embodiments the light source for illuminating thescale and the detector for detecting the scale can be located on thesame side of the scale. In such embodiments, the same (e.g. a single)outer case can comprise the light source and the detector.

Preferably the outer case provides solid particle protection to at leastlevel 4, and liquid ingress protection to at least level 4, according tothe International Protection Marking (also known an Ingress ProtectionMarking), International Electrotechnical Commission (IEC) standard60529. In other words, preferably the outer case has an IP rating of atleast IP44. The outer case could provide solid particle protection to atleast level 5, optionally to at least level 6. The outer case couldprovide liquid ingress protection to at least level 5, optionally to atleast level 6, for instance to at least level 7. In other words, theouter case could have an IP rating of IPxy where x (which relates tosolid particle protection) is at least 4 (e.g. 4 to 6) and y (whichrelates to liquid ingress protection) is at least 4 (e.g. 4 to 7).

This application describes a sealed encoder module for mounting onto amachine so as to measure relative displacement of first and second partsof the machine. As described the sealed encoder module can comprise, ascale, a readhead comprising a scale signal receiver, and a protectivehousing which encapsulates at least the scale and said scale signalreceiver. As described, the scale signal receiver can comprise an outercase within which components of the scale signal receiver are contained.According to a second aspect of the invention there is provided anencoder apparatus comprising a scale and a readhead assembly moveablerelative to each other, the scale and at least a scale signal receiverof the readhead assembly being located within a protective housing whichis configured to protect them from contamination located outside theprotecting housing and comprises a seal through which the scale signalreceiver can be connected to a part outside the protective housing. Thescale signal receiver can comprise an outer case within which componentsof the readhead are contained and protected from any contaminationpresent inside the protective housing. As explained above, the outercase could be hermetically sealed. Providing the scale signal receiverwith an outer case can help to ensure that one or more components of thescale signal receiver (i.e. component(s) for effecting the detection ofthe scale signal, e.g. electronic components and/or other componentsused for generating and/or interacting with, such as sensing and/ormanipulating, the signal from the scale) is/are protected even ifcontamination does manage to get inside the protective housing. This canimprove the reliability and longevity of the encoder apparatus. Such acomponent can comprise an electronic component. Such a component cancomprise a sensor. Such a component can comprise a component whichinteracts with the scale signal (e.g. used to manipulate the signal fromthe scale before it is sensed by the readhead's sensor). Such acomponent can comprise an emitter, such as a light emitter, e.g. forilluminating the scale. In the case that the encoder apparatus comprisesan optical encoder apparatus, the scale signal receiver's opticalcomponents can also be located inside said outer case. As will beunderstood, features explained above and below in connection with theother aspects of the invention are equally applicable to this aspect ofthe invention, and vice versa.

According to a third aspect of the invention there is provided areadhead assembly for an encoder apparatus (e.g. comprising at least onesensor for sensing scale features) comprising at least one vibrationcontrol device configured to reduce the susceptibility of at least apart of the readhead assembly (e.g. a scale signal receiving part) tovibrations. Accordingly, this application describes a readhead for anencoder apparatus comprising at least one sensor for sensing scalefeatures and at least one vibration control device configured to vibrateindependently of the rest of the readhead. Providing a readhead with atleast one vibration control device can control vibrations transferredthrough to it from the machine on which it is mounted. This isparticularly useful where the readhead is mounted to the machine via astructure that is susceptible to vibration (e.g. via an elongatemember). Optionally, the encoder apparatus comprise a sealed encoderapparatus. This need not necessarily be the case; for example theencoder apparatus could comprise an open/exposed encoder apparatus. Aswill be understood, features explained above and below in connectionwith the other aspect of the invention are equally applicable to thisaspect of the invention, and vice versa. Accordingly, for example, thereadhead could comprise a scale signal receiver. The scale signalreceiver could comprise the at least one vibration control device. Inthose embodiments in which the scale signal receiver is mounted viaelongate blade, the scale signal receiver and/or elongate blade couldcomprise at least one vibration control device.

According to a fourth aspect of the invention there is provided amachine comprising an encoder apparatus and/or readhead as describedherein.

According to another aspect of the invention there is provided anencoder apparatus, substantially as described herein and/or withreference to FIGS. 2 to 9.

According to a yet further aspect of the invention, there is provided, asealed encoder module for mounting onto a machine so as to measurerelative displacement of first and second parts of the machine. Thesealed encoder module can comprise a scale, a readhead comprising ascale signal receiver and an integral protective housing whichencapsulates at least the scale and said scale signal receiver. Thesealed encoder module can be configured to determine and outputdiagnostic information regarding a scale signal detected by thereadhead. As will be understood, the encoder module (e.g. the readhead)can also be configured to determine and output information concerningthe relative position of the scale and readhead. Accordingly, theencoder module can be configured to determine and output both positionand diagnostic information. As will be understood, features explainedabove and below in connection with the other aspects of the inventionare equally applicable to this aspect of the invention, and vice versa.

Accordingly, this application describes, an encoder apparatus comprisinga scale and a readhead moveable relative to each other, the scale and ascale signal receiver of the readhead being located on a first side of aseal so as to protect them from contamination present on a second sideof the seal, the scale signal receiver being rigidly fixed to a rigidreadhead mount which passes through the seal. Accordingly, the positionand orientation of the scale signal receiver on the first side of theseal can be dictated by (and mastered to) the readhead mount.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the following drawings, in which:

FIG. 1a schematically illustrates a prior art sealed encoder;

FIG. 1b schematically illustrates the prior art sealed encoder of FIG.1a with part of the protective housing cut-away to show the scale andscale sensor assembly located inside the protective housing;

FIG. 1c is a cross-section through the sealed encoder apparatus of FIG.1 a;

FIG. 1d schematically illustrates the prior art sealed encoder of FIG.1a with part of the protective housing cut-away to show the scale andscale sensor assembly located inside the protective housing;

FIGS. 2a and 2b are schematic illustrations of a sealed encoderapparatus according to the present invention, with part of theprotective housing cut-away to show the scale and scale signal receiverlocated inside the protective housing;

FIG. 2c is a cross-section through the sealed encoder apparatus of FIGS.2a and 2 b;

FIG. 2d is a cross-section through an alternative embodiment of anenclosed encoder apparatus;

FIG. 3 is an illustration of an alternative embodiment of a readheadassembly suitable for use with a sealed encoder, with part of the signalreceiving module cut-away to expose its internal components;

FIG. 4 is an illustration of the signal receiving module of the sealedencoder apparatus of FIG. 3;

FIG. 5 is an illustration of a tuned mass damper used in the signalreceiving module of FIGS. 3 and 4;

FIGS. 6a and 6b illustrate an alternative way of implementing avibration control device on a readhead assembly;

FIGS. 7a and 7b illustrate yet another way of implementing a vibrationcontrol device on a readhead assembly;

FIGS. 8a to 8c schematically illustrate further ways of implementing avibration control device; and

FIGS. 9a and 9b schematically illustrate rotary embodiments of theinvention.

Referring to FIGS. 2a to 2d there is a sealed encoder module 102according to the invention. The sealed encoder module 102 comprises ascale 104 having a plurality of features (not shown) and a readheadassembly 103, comprising a scale signal receiver 106 for receiving asignal from the scale. In the embodiment described the sealed encodermodule 102 is an optical encoder, in that the readhead assembly 103utilises electromagnetic radiation (EMR) in the infra-red to ultravioletrange in order to read the scale 104. In particular, in this describedembodiment, the position measurement encoder apparatus is an opticalabsolute encoder. Accordingly, the scale comprises a continuous seriesof uniquely identifiable features, e.g. codes, which the readheadassembly 103 can read and process to determine a unique position alongthe length of the scale 104. However, as will be understood, theposition measurement encoder apparatus need not necessarily be anabsolute encoder. For example, it could be an incremental opticalencoder. Furthermore, the encoder apparatus need not be an opticalencoder, for example, the encoder apparatus could be a magnetic encoder,or for instance an inductive encoder.

The readhead assembly 103 communicates with an external processor device(not shown), e.g. a controller, via a communications channel which inthe described embodiment comprises a physical connection (e.g. cable105) as opposed to a wireless connection. The communication channel canbe two-way such that the readhead assembly 103 can receive data (e.g.instructions) from the external processor device as well as send data(e.g. position information/signals) to the external processor device.Power to the readhead assembly 103 can also be supplied via a physicalconnection, e.g. via the cable 105. However, this need not necessarilybe the case. For example, the readhead assembly 103 could comprise aninternal power source such as a battery.

The scale 104 and scale signal receiver 106 are located inside aprotective housing 108 which protects them from contaminants external tothe protective housing. The scale 104 is fixed to the protective housing108 whereas the scale signal receiver 106 can move along the length ofthe scale 104 within the protective housing 108. In use, the protectivehousing 108 will be secured to a first part of a machine (not shown) andthe scale signal receiver 106 will be secured to a second part of themachine (not shown). As will be understood, the first and second partsof the machine are relatively moveable with respect to each other. Thereadhead assembly further comprises a mounting block 114 which is to bedirectly fastened to the second part of the machine (e.g. via one ormore releasable fasteners, such as threaded bolts passing through holes115), and a blade 116 which is connected to and extends between themounting block 114 and the scale signal receiver 106. A light source 113is provided on one end of the mounting block 114 and is used (asexplained in more detail below in connection with the other embodimentsof the invention) to relay diagnostic information concerning the encoderto an operator/installer.

The protective housing 108 further comprises a seal 111 in the form of apair of sealing lips 112 which seals the inside of the protectivehousing 108, in which the scale 104 and scale signal receiver 106reside, from external contaminants. The blade 116 passes between thepair of sealing lips 112. The sealing lips 112 are compliant so as to beable to part so as to allow the movement of the blade 116 and hence thescale signal receiver 106 along the length of the protective housing 108and hence the scale 104, but are also sufficiently elastic so as toclose together around the blade 116, thereby forming a physical barrierto solid and fluid (in particular liquid and moisture) contaminants. Inother words, the blade 116 prises the sealing lips 112 apart as it movesalong the length of the seal, between the sealing lips 112, and thesealing lips have sufficient elasticity so as to close together in theabsence of the blade 116.

Unlike the embodiment described above in connection with FIG. 1, in thiscase the arrangement of the scale signal receiver 106 within theprotective housing is independent of the scale 104 or the protectivehousing 108. The scale signal receiver 106 is rigidly connected to themounting block 114. In particular, scale signal receiver 106 is rigidlyconnected to the blade 116, which in turn is rigidly connected to themounting block 114. Accordingly, the position of the scale signalreceiver 106 in all degrees of freedom is dictated by the position ofthe mounting block 114 and hence dictated by the position of the secondpart of the machine to which the mounting block 114 is secured duringuse, and not by the scale 104 or other part inside the protectivehousing 108.

Accordingly, in contrast to the embodiment described in connection withFIG. 1, in the embodiment described the scale signal receiver's 106position and motion is not constrained or guided in any way by the scale104 or protective housing 108. Due to the rigid mount between the scalesignal receiver 106 and the mounting block 114 the position and motionof the scale signal receiver 106 in all six degrees of freedom isconstrained and guided by the position and motion of the mounting block114, and hence the part of the machine to which the mounting block 114is secured. Accordingly, the position and motion of the scale signalreceiver 106 could be described as being “externally constrained” (incontrast to the scale sensor assembly 6 of FIG. 1 in which the positionand motion of the readhead 6 is “internally constrained”).Additionally/alternatively, the sealed encoder module 102 could bedescribed as being a “bearingless” or as a “without integral bearing”encoder (in contrast to the encoder module 2 of FIG. 1 which could bereferred to as an “integral bearing” encoder).

As will be understood, if desired, an adjustment mechanism could beprovided for adjusting the relative set-up position of the scale signalreceiver 106 with respect to the mounting block 114 (e.g. the scalesignal receiver could be connected to the blade 116, and/or the blade116 could be mounted to the mounting block 114, via a joint whichfacilitates selective adjustment of their relative position in at leastone linear and/or one rotational degree of freedom, for example bymanipulation of a micro/grub screw). Such a selective adjustmentmechanism could be useful to aid set-up/alignment of the encoderapparatus. However, as will be understood, such a selective adjustmentmechanism will still provide a rigid connection between the scale signalreceiver 106 and the readhead mount 114, and hence a rigid connectionbetween the scale signal receiver 106 and the part of the machine onwhich it is mounted (i.e. so that during use/operation, theposition/orientation of the scale signal receiver 106 in all degrees offreedom is still mastered to/dictated by the second part of the machineto which the mounting block 114 is mounted).

In the described embodiment, the scale signal receiver 106 does notcontact the scale 104, nor the protective housing 108 at all.Accordingly, there is a gap all the way around the scale signal receiver106, between it and the scale 104 and the inside of protective housing108. Indeed, as shown, in the embodiment described, the only contactbetween the readhead assembly 103 (which comprises the scale signalreceiver 106 and the readhead mount 110) and the protective housing 108is between the blade 116 and the pair of sealing lips 112. As will beunderstood, the pair of sealing lips 112 are flexible and elastic inbehaviour and yield to accommodate the blade 116, and thereby do notconstrain or control the position of the scale signal receiver 106.

Furthermore, in the embodiment described, the scale signal receiver 106comprises an outer case 107, inside of which is located the scale signalreceiver's electrical components. The scale signal receiver's 106 sensorfor detecting the scale signal coming from the scale 104, and also anyassociated components for forming the scale signal on the sensor (e.g.optical components such as a lens, diffraction grating and/or mirrors)can also be provided inside the scale signal receiver's outer case 107.The outer case 107, is configured (e.g. sealed) such that ifcontamination did inadvertently pass through the lip seals 112, then thescale signal receiver's 106 components (in particular the electrical andoptical components) inside the outer case 107 are protected.

As will be understood, in embodiments in which an outer case 107 isprovided, a window (e.g. window 232 in FIGS. 3 and 4) can be provided toenable the scale signal to reach the sensor located inside the outercase 107. Optionally, the window has no material effect on the scalesignal (e.g. its only purpose could be to merely allow the signal fromthe scale to enter the outer case 107 without contributing to the formof the signal received at the readhead's sensor). Optionally, the windowcould be configured to re-direct the signal coming from the scale (e.g.it could comprise a mirror). Optionally, the window could be configuredto interact with the signal from the scale so as to produce the desiredsignal detected at the sensor. For example, it could comprise adiffraction grating, and/or lens. In any case, as will be understood,the outside of the window 232 will not be sealed from any contaminationentering the protective housing 108, since it forms part of the outercase 107, but the inside of the window, and any other components (e.g.optical components) which are configured to manipulate the signal comingfrom the scale 104 are protected from contamination.

The benefits of providing the scale signal receiver 106 with an outercase 107 can be beneficial not just for embodiments in which the scalesignal receiver 106 is independently arranged with respect to the scale104/protective housing 108 (e.g. which is rigidly mounted and“externally constrained”), but can also be beneficial for “integralbearing” encoders (e.g. those embodiments in which the scale signalreceiver is mounted to the readhead mount via an articulated linkage andthe position of which is “internally constrained”. For example, an outercase can also be beneficial in “integral bearing”/“internallyconstrained” enclosed encoders of the type described above in connectionwith FIG. 1. Accordingly, as will be understood, in connection with thisaspect, there could be provided an articulated linkage such as thatdescribed in connection with FIG. 1. However, although providing anouter case 107 can improve the resilience such “integralbearing”/“internally constrained” enclosed encoders, if contaminationdoes pass through the sealing lips 12 and lands on the scale, this canadversely affect the performance of the encoder apparatus. For example,if sufficient contamination landed on the scale features then this couldadversely affect the signal coming from the scale. Also, if solidcontamination such as swarf entered the protective housing and fell onthe track(s) along which the readhead's bearings 20 run, this couldadversely affect the relative position/orientation of the scale signalreceiver and scale as the bearings between the scale and scale signalreceiver rides over the dirt. Of course, an enclosed encoder with ascale signal receiver arranged independently of the scale (e.g.“externally constrained”) has the additional benefit of not sufferingfrom such a problem.

As explained in more detail below in connection with the otherembodiments of the invention, the scale signal receiver 106 receives asignal from the scale which is processed in order to provide, via cable105 for example, a position signal to an external device (such as amachine controller). For example, processing to determine the positioncould be performed by one or more processor devices in the scale signalreceiver 106, and/or by one or more processor devices in another part ofthe readhead assembly such as the mounting block 114. Optionally, theblade 116 comprises one or more channels to enable wires to pass betweenthe scale signal receiver 106 and the mounting block 114. Alternatively,wireless communication could be used, or wired connections external tothe blade 116 could be used. If the blade 116 comprises one or morechannels, then air (for example supplied via an air supply line 109)could be passed through to the inside of the protective housing 108 viathe blade 116 (e.g. via holes in the blade 116).

As will be understood, FIGS. 2a to 2d are schematic and typically theseparation between the scale 104 and scale signal receiver 106 (oftenreferred to as the ride-height) can be much smaller than that shown. Thedesired ride-height will depend on the encoder, but for example, typicalride-heights for optical encoders can be in the region of 0.24 mm to 2mm. In the particular example described, the nominal ride-height is 0.8mm, with a +/−0.15 mm tolerance.

The sealed encoder module 102 shown in FIGS. 2a to 2d can be used in anyorientation. In FIGS. 2a to 2d , the mounting block 114 is shown to bepositioned directly above the scale signal receiver 106 and theprotective housing 108. However, this need not necessarily be the case.For example, the sealed encoder module 102 could be mounted on its side,or even upside down (such that the mounting block 114 is positioneddirectly below the scale signal receiver 106 and the protective housing108). Indeed, such an arrangement can be advantageous because anyexternal contamination will tend to fall away from the lip seals 112 ofthe protective housing 108 due to gravity.

Likewise, the pair of sealing lips 112 need not be provided directly onthe side of the protective housing 108 that is opposite the side of theprotective housing 108 on which the scale is located. For example, withreference to the orientation shown in FIG. 2c , the sealing lips 112could be provided on one of the vertical sides of the protective housingsuch that the blade 116 extends horizontally as opposed to vertically.Alternatively, they could be provided along one of the corners/edges ofthe protective housing between two sides, such as shown in FIG. 2d(which as shown in this embodiment the seal 111 comprises two pairs ofsealing lips 112).

Referring now to FIGS. 3 to 5, there is shown another readhead assembly203. The readhead assembly 203 of FIGS. 3 to 5 shares many similaritieswith the readhead assembly 103 of FIG. 2 and for instance comprises ascale signal receiver 206, a mounting block 214, a light emitter 213,and a blade 216 providing a rigid connection between the scale signalreceiver 206 and the mounting block 214 (accordingly, the scale signalreceiver 206 is “externally constrained”). FIG. 3 shows the readheadassembly 203 in isolation, but as will be understood the readheadassembly 203 is intended to be used to read a scale that is locatedinside a protective housing, like that shown in FIGS. 2a to 2d .Accordingly, it is also intended that the scale signal receiver 206 willalso be located inside the protective housing, and the blade 216 willpass through an elongate seal in the protective housing, such as a pairof sealing lips. As with the embodiment of FIGS. 2a to 2d , the scalesignal receiver 206 is an optical readhead, but this need notnecessarily be the case.

As with the scale signal receiver 106 of FIG. 2, the scale signalreceiver 206 of FIGS. 3 and 4 comprises a protective outer case 207. Inthis case, the components inside the scale signal receiver 206 areprotected (e.g. sealed) by way of the protective outer case 207 and amounting face 217 provided at the end of the blade 216 proximal thescale signal receiver 206 via which the scale signal receiver 206 ismounted to the blade 216. A sealing member can be provided at theinterface between the outer case 207 and the mounting face 217 (e.g. agasket could be sandwiched between the outer case 207 and the mountingface 217 of the blade 216).

As shown, rather than the blade 216 extending perpendicularly betweenthe scale signal receiver 206 and the mounting block 214 (as in theconfiguration of FIG. 2), in this embodiment the blade extends at anon-perpendicular angle, for example approximately 45 degrees betweenthe scale signal receiver 206 and the mounting block 214. This is suchthat the blade can be oriented such that any liquid falling on it willfall away from the sealing lips, regardless of whether the sealedencoder module is mounted vertically or horizontally.

As shown in FIGS. 3 and 4 there is shown an optical unit 230 comprisingthe scale signal receiver's components for detecting the scale signal.In particular, the optical unit comprises a light source 252 forilluminating the scale, a lens 254 configured to image the scale, asensor 256 on which said image falls and is configured to detect saidimage (e.g. a one or two dimensional CCD or CMOS sensor), and a beamsteerer 258 which is configured to direct light from the light sourceonto the scale. As shown, the sensor 256 can be mounted on a printedcircuit board (PCB) 240. A cable (not shown) connects the PCB 240 to aprocessor device inside the mounting block 214. When an image isobtained by the sensor, it is passed to the processor device locatedinside the mounting block 214, which processes the image to determine aposition (in a known manner, e.g. as explained in US2012072169, thecontent of which is incorporated herein by this reference). Thedetermined position is then communicated to an external device (such asa machine controller for example), for example via one or more signalstransmitted along cable 205. As will be understood, other arrangementsare possible. For example, all processing could be performed by one ormore processor devices located in the scale signal receiver 206. Inanother alternative embodiment, the sensor device (e.g. a CCD or CMOS)could be located in the mounting block and could receive the scalesignal via a light guide (e.g. fibre optic) that extends through theblade 216. Accordingly, in this case the scale signal receiver 206merely collects the signal/light from the scale and passes it through toa sensor located elsewhere in the readhead assembly.

As mentioned above, a light emitter 213 (113 in the embodiment of FIGS.2a to 2d ) for relaying diagnostic information can be provided by theencoder module; for example by the readhead assembly. Such a lightemitter can be used to relay diagnostic information to anoperator/installer. For example, the colour and/or brightness of lightemitted by the light source controlled so as to replay diagnosticinformation. Optionally, the light emitter could be configured to flashin particular ways so as to relay diagnostic information.

For example, the light emitter could be controlled so as to emit avisual signal that is dependent on the relative set up of the readhead(e.g. scale signal receiver) and the scale. This could be particularlyuseful during installation of the encoder module so as to confirm thatthe readhead is receiving a good signal from the scale. For instance,the encoder module could be configured such that the colour of the lightemitter 213 is dependent on the relative set up (e.g. green light couldbe emitted when the readhead is receiving a good/strong scale signal,and red light could be emitted when the readhead is receiving apoor/weak scale signal). Such a visual indication for indicating therelative set up of the readhead and scale can be useful for both“independently arranged” and “internally constrained” encoder apparatus.Such a visual indication for indicating the relative set up of thereadhead and scale can be particularly useful when (as mentioned above)an adjustment mechanism is provided for adjusting the relative set-upposition of the scale signal receiver with respect to the mountingblock.

In the embodiment described, the processor inside the mounting block 214that is used to determine a position is also configured to process theimage detected by the sensor 256 in order to determine the diagnosticinformation (however as will be understood this need not necessarily bethe case; a separate processor could be used). In the embodimentdescribed, the processor is configured to determine diagnosticinformation based on the quality of the signal detected by the sensor.In this particular embodiment, it is configured to Fourier Transform theimage obtained by the sensor at the fundamental spatial frequency, ω, ofthe scale's features (which could be provided during set up of theencoder module or by analysis of the image). The magnitude, A, of theFourier transform is then established. As will be understood, a Fouriertransform provides a real part

and an imaginary part 3, and the magnitude A can be calculated from thefollowing equation:

A=√{square root over ([

(F(ω))]²+[

(F(ω))]²)}or A ²=[

(F(ω))]²+[

(F(ω))]²  (1)

-   -   where F(ω) represents the Fourier transform of the        representation at spatial frequency ω

Since computing a square root is computationally intensive, it will beunderstood that it may be preferable to use A² instead of A to determinethe setup indicator output. The method then comprises comparing the A(or A²) to threshold values to determine how to control the lightemitter 213. For example, when A (or A²) has a value below a thresholdthen the light emitter can be controlled to output red light and when A(or A²) has a value above a threshold then the light emitter can becontrolled to output green light.

As will be understood, A (or A²) is dependent on the amplitude of thefeatures as obtained in the representation. This is in turn affected bythe setup of the readhead relative to the scale (which is what is to bedetermined). A (or A²) is also dependent on the number of features inthe representation. Accordingly, if there is significant variation inthe density of features along the scale, then the method can comprisesteps to compensate for this. For example, this compensation may beachieved by dividing A (or A²) by the number of features in therepresentation.

In the described embodiment, the method involves Fourier Transformingthe representation substantially at the fundamental spatial frequency ofthe features. The Fourier Transform could use an assumed fundamentalspatial frequency of the features, based on the scale that it is beingused with. Even if the assumed fundamental frequency is not exactlycorrect, then the method can still provide a useful indication of thequality of the representation. Optionally, the fundamental spatialfrequency of the features could be determined by analysing the imagebefore performing the Fourier Transform. This could be useful inembodiments in which the actual fundamental spatial frequency of thefeatures as imaged varies significantly due to rideheight/magnificationeffects.

Furthermore, as will be understood, it need not necessarily be the casethat the Fourier Transform is performed substantially at the fundamentalspatial frequency of the features. For instance, the method couldinvolve performing the Fourier Transform at some other frequency, e.g.at a harmonic of the spatial frequency. Optionally, the method couldinvolve performing the Fourier Transform at one or more frequencies andcomparing the magnitude of the Fourier Transforms at the differentspatial frequencies.

Additional details of how an image of an absolute scale can be processedto determine diagnostic information is described in U.S. Pat. No.8,505,210, the content of which is incorporated herein by thisreference. As will be understood, there are other ways in which thediagnostic information can be determined. For example, as described inU.S. Pat. No. 8,505,210, the relative amplitude of different types ofscale features as imaged can be determined which can be indicative ofthe quality of the scale signal detected.

As shown, in this embodiment, the scale signal receiver 206 alsocomprises a vibration control device (in fact, this embodiment comprisesa plurality of vibration control devices), which in this particularembodiment comprises a tuned mass damper 260. Our inventors have foundthe use of at least one vibration control device can improve the lifeand/or metrological performance of an encoder apparatus. This isparticularly the case when the scale signal receiver is rigidly mountedto a structure via a member susceptible to vibration (e.g. a memberwhich transmits and/or amplifies vibration) such as an elongate arm or athin blade to which the scale signal receiver is rigidly mounted. Forexample, in the case of the “externally constrained” scale signalreceiver of the embodiments described above, vibrations are passedthrough to the scale signal receiver via the rigid mounting arrangement.A vibration control device provides a way of controlling such unwantedvibration to which the scale signal receiver is exposed.

As will be understood, a vibration control device can be a deviceconfigured to reduce the response of a system (e.g. the scale signalreceiver) due to external excitation. As mentioned above, in thisparticular example, the vibration control device comprises a tuned massdamper 260 which is tuned so as to reduce the amplitude of vibrations inthe system in which it is installed, at and around the system's resonantfrequency. As will be understood, a tuned mass damper comprises aspring, a damper and a mass. The spring's stiffness “k”, the damper'sdamping coefficient “c” and the mass's mass “m” are selected (in otherwords “tuned”) so as to reduce the amplitude of vibrations of the systemin which it is installed, at and around the system's resonant frequency.In this embodiment, the tuned mass damper comprises a pair of elastomerrings 262 (for example rubber rings), which provides the spring anddamper elements, and a body 264 which provides the mass element.Accordingly, each elastomer ring 262 acts as a spring and a damper, byway of absorbing energy and converting the energy to heat. The body 264comprises a sufficiently dense material (e.g. brass) so as to enable thebody 264 to have sufficiently small size whilst providing suitable highmass. Typically, the mass of a tuned mass damper needs to be asubstantial percentage of the mass of the system it is intended to damp(in this case the parts of the readhead assembly located inside theprotective housing, in particular the scale signal receiver 206). Forexample, in this case, the mass of the tuned mass damper 260 can be atleast 1% of the mass of the scale signal receiver 206, optionally atleast 2% of the mass of the scale signal receiver 206, for exampleapproximately 5% of the mass of the scale signal receiver 206. Forexample, in this case, the mass of the tuned mass damper 260 could beconfigured such that it is not more than 30% of the mass of the scalesignal receiver 206, optionally not more than 25% of the mass of thescale signal receiver 206.

As shown in FIG. 4, the tuned mass dampers 260 are located insidecylindrical holes provided by the scale signal receiver 206. Althoughnot shown, in the particular embodiment described, the sides of thecylindrical holes comprises a plurality of elongate, axially extendingridges (or “splines”) such that the outer circumference of the elastomerrings 262 engages said ridges, thereby reducing the contact area betweenthe elastomer rings 262 and the inside of the hole. This helps to keepdown the stiffness of the elastomer rings 262, which in turn helps toreduce the natural frequency of the tuned mass dampers 260. Such aconfiguration avoids the need to use a greater mass 264 or softerelastomer rings 262 to obtain the desired damping effect.

As will be understood, the elastomer rings 262 and the cylindrical holein which the tuned mass dampers 260 are located could be shaped andsized such that the elastomer rings 262 are squashed/compressed withinthe holes. As will be understood, even in such a case, the mass element264 will move/vibrate around independently of the scale signal receiver206. Alternatively, the elastomer rings 262 and the cylindrical hole inwhich the tuned mass dampers 260 are located could be shaped and sizedsuch that the elastomer rings 262 are not squashed/compressed within theholes. Accordingly, the elastomer rings 262 and the cylindrical hole inwhich the tuned mass dampers 260 are located could be shaped and sizedsuch that the elastomer rings 262 are free to rattle/bounce aroundwithin the holes.

FIGS. 6a, 6b, 7a and 7b illustrate further alternative implementationsof suitable vibration control devices. With respect to FIGS. 6a and 6b ,the vibration control device comprises a mass element 364 connected tothe outer case 207 of the scale signal receiver 206 via a spring anddamper element 362. In this case the spring and damper element 362 is ablock of elastomer material, such as rubber. The mass element 364 istherefore able to vibrate independently of the scale signal receiver206, by virtue of the flexibility of the spring and damper element 362(which acts as a spring and a damper, by way of absorbing energy andconverting the energy to heat).

FIGS. 7a and 7b illustrate another alternative embodiment comprising atuned mass damper 460. In this case the tuned mass damper 460 comprisesa mass 464 formed as an integral part (e.g. via a single moulding) ofthe outer case 207 of the scale signal receiver 206. The tuned massdamper also comprises a spring element 466 which is also formed as anintegral part of the outer case 207 of the scale signal receiver 206. Asshown in the cross-sectional drawing of FIG. 7b , the material of thespring element 466 provided by the outer case 207 is sufficiently thinso as to be flexible enough to enable the mass 464 to move and vibraterelative to the rest of the scale signal receiver 206. In thisembodiment, a separate damping element 462 (shown in FIG. 7b ) isprovided, which comprises an elastomer ring 462 that extends around atrough in the outer case 207 resulting from the presence of theintegrally formed spring element 466.

As will be understood, FIGS. 6b and 7b also illustrate how that theblade 216 can be hollow for the passage of wires (not shown) and/or air(as explained above). These figures also show how that the mountingblock 214 can comprise space for components such as at least oneprocessor device 242 (as explained in more detail above).

As schematically illustrated by FIG. 8a , the spring and damper parts ofthe tuned mass damper need not be provided by a common part. Forexample, an example tuned mass damper 560, can comprise a mass 562, andone or more (in this case four) springs 566 (which have little or nosubstantial damping effect) and one or more (in this case four) dampingelements 564.

In the above described embodiments, the vibration control devicecomprises a tuned mass damper. However, as will be understood, this neednot necessarily be the case. For example, the vibration control devicecould comprise a vibration absorber 660, an example of which isillustrated in FIG. 8b . As schematically illustrated, a vibrationabsorber 660 can comprise a mass element 662, and one or more springs666 (in this example four springs 666) which enable the mass 662 tomove/vibrate independently of the outer case 207 and the rest of thescale signal receiving unit 206.

In the embodiments depicted in FIGS. 8a and 8b , the vibrationcontrollers 560, 660 are located in a recess provided in the outer case207 of the scale signal receiver 206, but as will be understood otherarrangements are possible. For example, as shown in FIG. 8c thevibration controller 760 (comprising a mass element 762, spring 766 andoptionally a damper element 764) could be connected to the side of theouter case 207 of the scale signal receiving unit 206.

In the above described embodiments, the encoder and scale are linear.However, as will be understood, the invention is equally applicable tonon-linear encoders/scale, for example rotary encoders such as discand/or ring encoders. FIGS. 9a and 9b schematically illustrate exampleimplementations of such embodiments. In the embodiment of FIG. 9a , thescale 804 is provided on the face of a disc (shown as a dashed line) andis contained within a cylindrical protective housing 808. A circularseal 811, through which the blade 216 of a readhead assembly can pass,is provided on the end face of the cylindrical protective housing 808(although as will be understood could be provided on the cylindricalside face of the cylindrical protective housing 808 if desired). In theembodiment of FIG. 9b , the scale 904 is provided on the circumferentialside of a ring (shown as a dashed line) and is contained within acylindrical protective housing 908. A circular seal 911, through whichthe blade 216 of a readhead assembly can pass, is provided on thecylindrical side face of the cylindrical protective housing 908(although as will be understood could be provided on the end face of thecylindrical protective housing 908 if desired). In these embodiments,the readhead assembly (comprising the scale signal receiver 207,mounting block 214 and blade 216) can be the same as described above(although in the embodiment of FIG. 9a , it might be beneficial for theblade to be curved to follow the curvature of the seal 811). In boththese embodiments, a light emitter 213 is provided on the mounting block214 and the encoder is configured to control the light emitter to relaydiagnostic information.

In the embodiments described above, the readhead assembly comprises ascale signal receiver 106, mounting block 114 and a blade 116. However,as will be understood, the readhead assembly could comprise a scalesignal receiver 106 only. For example, the blade could be provided bythe machine on which the scale signal receiver 106 is to be mounted. Forexample, in connection with the above described embodiments, the sealedencoder module could be supplied without a mounting block 114 and/orblade 116, but rather just the scale signal receiver 106 which is (or isto be) located inside the protective housing 108. During set up, thescale signal receiver 106 can be connected to a blade or equivalentwhich is provided by the machine on which the encoder apparatus is beinginstalled.

In the above described embodiments, the encoder is a reflective opticalencoder (e.g. the readhead detects the scale by light reflected from thescale, and the readhead's light source and detector(s)/sensor(s) arelocated on the same side of the scale). As will be understood, theencoder could be a transmissive optical encoder (in which case thereadhead's light source and detector(s)/sensor(s) are on opposite sidesof the scale). As will also be understood, the invention is applicableto non-optical encoders (e.g. magnetic, inductive and/or capacitiveencoders).

As described above, the scale comprises features which are used toprovide a signal detectable by the readhead assembly's sensor. In theembodiments described above, the encoder/scale comprises an absoluteencoder/scale. The readhead decodes the image obtained to determine anabsolute position. However, this need not necessarily be the case. Forexample, the encoder/scale could be an incremental encoder/scale (withor without reference marks). As is well known, the readhead could beconfigured to output quadrature signals which can be used to determinerelative motion and/or position of the scale and readhead. In this case,an alternative technique could be used to determine diagnosticinformation that can be used to determine how to control the lightemitter 113, 213. For example, the encoder module (e.g. the readhead)could be configured to determine whether the quadrature signal levelsare above or below given threshold levels to determine how to controlthe light emitter 113, 213. Further details of such a process aredescribed in U.S. Pat. No. 5,241,173, the content of which isincorporated herein by this reference.

The encoder could be diffraction-based, e.g. the signal detected by thescale sensor assembly's sensor is formed by the scale (and one or morediffraction gratings in the scale sensor assembly) diffracting light(e.g. forming an interference fringe at the scale sensor assembly'ssensor).

As will be understood, references to light in this application compriseelectromagnetic radiation (EMR) in the ultra-violet to infra-red range.

In the above described embodiments, a vibration control device is usedto reduce the susceptibility of the scale signal receiver to vibrations.However, as will be understood, a vibration control device is optional.Indeed, a vibration control device might be unnecessary depending on thefrequency of the vibration the encoder is to be exposed to and theresonant frequency of the scale signal receiver. Optionally, anyvibrations induced in the scale signal receiver could be sufficientlysmall so as to not affect the structural stability of the scale signalreceiver and/or produce measurement errors which are within desiredtolerances.

In the above described embodiments, the scale signal receiver comprisesan outer casing which encapsulates the scale signal receiver components.However, this need not necessarily be the case. For example, theelectronic and/or other (e.g. optical) components could be exposed. Forexample, the PCB 240 could be exposed within the protective housing 108.

In the embodiments described, a light emitter 113, 213, is provided onthe readhead to relay diagnostic information determined by the encoder.However, as will be understood, this need not necessarily be the case.For example, as illustrated in FIGS. 2a, 2b, 9a, 9b , a light emitter113′, 213′ could be provided on the protective housing 108, 808, 908instead of/in addition to the light emitter on the readhead. In thiscase, the protective housing could comprise an internal power source(e.g. a battery) for powering the light emitter and/or could beconnected to an external power source. Furthermore, the protectivehousing could be configured to receive diagnostic information from thereadhead in order to determine how to control the light emitter.Optionally, the protective housing is configured to receive the scalesignal detected by the readhead and is configured to determine thediagnostic information itself in order to determine how to control thelight emitter. Either way, the protective housing could comprise its ownprocessor device configured to determine how to control the lightemitter (e.g. in response to the diagnostic received and/or subsequentto it determining the diagnostic information itself).

Furthermore, in other embodiments, additionally or alternatively to sucha light emitter being provided, the encoder module could be configuredto determine and output diagnostic information in the form of one ormore electronic signals to an external device (e.g. a controller), forexample via cable 105, 205. For instance, diagnostic informationconcerning the quality of the scale signal detected by the readheadcould be determined and output by the encoder module. The externaldevice receiving this information could, for example, display thisinformation to an operator. Such diagnostic information could be usefulto help an operator determine the status of the encoder module, e.g. todetermine if the encoder module is operating properly and take action ifit is not (e.g. stop the machine on which the encoder module isinstalled and/or replace the encoder module).

As will be understood, the capability of determining and outputtingdiagnostic information is optional.

As will also be understood, a bracket (e.g. a “transit bracket”) or thelike can be used to keep the readhead assembly and the protectivehousing in a predetermined physical relationship, e.g. such as when theyare not mounted on a machine.

1. A readhead for an encoder apparatus comprising at least one vibrationcontrol device configured to vibrate independently of the rest of thereadhead and configured to reduce a susceptibility of the readhead tovibrations.
 2. A readhead as claimed in claim 1, in which the vibrationcontrol device is connected to only a single unitary body of thereadhead.
 3. A readhead as claimed in claim 1, in which the vibrationcontrol device is configured to reduce the amplitude of vibrations ofthe readhead at and around the readhead's resonant frequency.
 4. Areadhead as claimed in claim 1, in which the vibration control deviceresides within the readhead.
 5. A readhead as claimed in claim 4, inwhich the vibration control device resides within an outer case of thereadhead.
 6. A readhead as claimed in claim 4, in which the vibrationcontrol device resides within a void defined by the readhead and isconfigured to vibrate within said void independently of the rest of thereadhead.
 7. A readhead as claimed in claim 5, in which the vibrationcontrol device resides within a void defined by the readhead and isconfigured to vibrate within said void independently of the rest of thereadhead.
 8. A readhead as claimed in claim 1, in which the at least onevibration control device comprises a cylindrical body with a firstelastomer ring disposed on one end of the cylindrical body, and a secondelastomer ring disposed on another end of the cylindrical body.
 9. Areadhead as claimed in claim 1, in which the at least one vibrationcontrol device comprises at least one member which is configured with aresonant frequency independent of the readhead.
 10. A readhead asclaimed in claim 1, in which the at least one vibration control devicecomprises one or more spring elements, one or more mass elements, andone or more damper elements.
 11. A readhead as claimed in claim 10, inwhich at least one of the one or more spring elements, at least one ofthe one or more mass elements, and at least one of the one or moredamper elements are provided by a single spring mass damper element. 12.A readhead as claimed in claim 1, in which the at least one vibrationcontrol device comprises a tuned mass damper.
 13. A readhead as claimedin claim 1, in which the readhead comprises at least one scale signalreceiver part for receiving a signal from a scale, in which the scalesignal receiver part comprises said vibration control device.
 14. Areadhead as claimed in claim 13, in which the readhead comprises a mountpart comprising mounting features for securing the readhead to a part ofa machine, and in which the scale signal receiver part of the readheadis rigidly connected to the mount part.
 15. An encoder apparatuscomprising a scale and a readhead as claimed in claim
 1. 16. An encoderapparatus as claimed in claim 13, in which the scale and the scalesignal receiver are located within a protective housing which isconfigured to protect them from contamination located outside theprotective housing, the scale signal receiver and the protective housingbeing relatively moveable with respect to each other, the protectivehousing comprising a seal through which the scale signal receiver can beconnected to a part outside the protective housing.
 17. An encoderapparatus as claimed in claim 15, in which the scale and a scale signalreceiver are located within a protective housing which is configured toprotect them from contamination located outside the protective housing,the scale signal receiver and the protective housing being relativelymoveable with respect to each other, the protective housing comprising aseal through which the scale signal receiver can be connected to a partoutside the protective housing.